Age-related decline in physical health is one of the most pressing concerns our country faces. In the context of this important societal challenge, one of the major contributions of health psychology has been to demonstrate that variation in physical health is reliably associated with aspects of the person (i.e., individual differences in personality traits) as well as the key social contexts in which the person is embedded (i.e., the quality and nature of adults? interpersonal experiences). For example, in predicting mortality, the effect size for the personality traits of conscientiousness and neuroticism are equal to, if not greater than low SES or low IQ1. Similarly, it has been noted that ?social relationships?constitute a major risk factor for health?rivaling the effect of well established health markers such as cigarette smoking, blood pressure, blood lipids, obesity and physical activity?2 (p. 1). In short, that key intra- and interpersonal resources are reliably associated with health status is now not in dispute. Nevertheless, the degree to which these associations emerge from environmentally mediated processes amenable to intervention remains largely unknown because existing research cannot rule out the possibility that some of these associations might reflect the impact of confounds, including especially evocative genetic processes, that produce correlations between personality/interpersonal experiences and physical health3, 4. We propose to use an innovative co-twin control design to rule out potential genetic and family background confounds and thereby identify environmentally mediated connections linking both personality and interpersonal experiences in advancing age with physical health outcomes. To sidestep problems with correlated measurement error, we will use a multiple-levels-of-analysis approach to assessment, including: (a) self- and romantic partner-reports of personality, social support, and relationship satisfaction, (b) observations of marital interactions while participants? autonomic nervous system responses are monitored continuously, and (c) measurement of health-related behaviors and both standard and early biological markers of health status. Moreover, we intend to do all of this in the context of a genetically informed research design. Though this has never before been done, it is critical in properly targeting interventions to improve health among older adults. More specifically, we will study whether the relation between individual/interpersonal differences and physical health is environmentally mediated by working with participants in the Minnesota Twin Registry (MTR) now in late life (800 twin pairs with their romantic partners, as applicable), a group that completed an assessment of personality previously (~28 years ago, when they were ~38 years old). The acquisition of these new measures from the MTR will allow us to address our specific aims: (Aim 1) to determine whether associations between both personality and relationship experiences in late life and physical health are environmentally mediated, (Aim 2) to identify mechanisms in the environmental pathways connecting personality and health, and (Aim 3) to test whether the effects of personality and relationship qualities extend to biological indicators of health risk.